Date: Mon Aug 3, 1998
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** This section is a how-to guide for making cast resin parts.
[Q] How do I cast my own parts in resin?
[A] firstname.lastname@example.org (James Wentworth 2/97)
[In addition to James' excellent article below, you might want to
check the url: www.inetc.roland.co.jp/~adrian/moldcast/moldcast.html
- Don Schmitz]
I have seen many questions about resin casting on rms, so I thought I'd
send you this guide. It is geared toward model rocketry-related casting
situations, but the techniques described can also be used to cast parts
for non-flying models. Here it is:
RESIN CASTING FOR ROCKETEERS
Nose cones, fins, transitions, and other model rocket parts can be easily
produced using resin casting techniques. The time and effort that go into
fabricating a custom nose cone or a built-up fin need only be expended
once to produce a master pattern, around which an RTV
(room-temperature-vulcanizing) rubber mold will be poured. The mold can
then be used to cast many duplicates of the original part using two-part
room-temperature-curing polyurethane resin.
This article is not intended to be a comprehensive guide, but rather a
primer that will cover most of the unique molding and casting situations
that model rocketeers will encounter. Although the RTV rubbers and
resins are neither corrosive nor highly toxic, it is advisable to use
them only in a well-ventilated area away from open flame or sparks.
Before using the materials, read and understand the Material Safety Data
Sheets and the manufacturer's safety instructions that are included with
the RTV rubbers and resins.
Creating A Master Pattern
To produce an RTV rubber mold, a high-quality master pattern must first
be made. A master can be made of any material that does not inhibit the
curing of the RTV rubber. Balsa, cardboard, hardwood, plastic, metal,
and sulfur-free "plasticene" modeling clay are suitable materials.
Porous materials such as balsa must be sanded and sealed before the RTV
can be poured over them. The master should also be clean and dry. RTV
silicone rubbers and polyurethane resins will not cure properly if
moisture is present on the master or mold.
To make a nose cone master, for example, a hardwood nose cone can be turned
on a lathe or a drill press, then sanded and sealed. The nose should be
sprayed with a wax-based release agent (Synair's Synlube 531, for
example) and allowed to dry. The dowel at the nose's rear can be pressed
into a smooth bed of sulfur-free "plasticene" modeling clay (other clays
can inhibit curing in the RTV rubber). The dowel is pressed into the
clay until the back of the tenon is flush with the clay surface. A
superior master can be made by turning polycarbonate, lucite, lexan,
nylon, or aluminum rod stock on a lathe. Masters made of these materials
require neither sanding & sealing nor a release agent. A transition or a
boat-tail would be produced in a similar fashion. Consult Peter Alway's
"The Art of Scale Model Rocketry" (Saturn Press, 1994) for more
information on turning parts.
Making The Mold Box
A mold box must be built around the master. It can be cardboard, wood,
metal, or plastic. Porous box materials must first be sealed with a
wax-based mold release or be covered with plastic tape like scotch tape.
An easy way to make a small mold box is to roll a piece of sheet styrene
into a cylinder, tape the seam, and stick it into the clay around the
master. There should be 1/4" to 1/2" of space between the master and the
walls of the mold box. The walls should be 1/4" to 1/2" higher than the
Mixing & Pouring The RTV Rubber
The RTV silicone rubber can now be mixed. RTV silicones consist of a
rubber and a catalyst, and most are mixed in a 10:1 ratio (rubber to
catalyst) by weight. For one-piece molds, use a low-durometer (soft)
rubber no harder than a Shore A15 (Ace Resin's RTV and Dow Corning's HS-III
RTV are A12). The rubber and catalyst are weighed (a postal scale is
good for this) and mixed in plastic containers (do NOT use wax-lined or
styrofoam cups). Pour the catalyst into the rubber and fold the rubber and
catalyst together, being careful not to whip air into the mixture.
Scrape the bottom and sides of the mixing container to make sure the
rubber and catalyst are thoroughly mixed. It is often helpful to
transfer the material into a second container to complete the mixing.
Any unmixed material could leave uncured sticky spots in the mold cavity.
If you have access to a vacuum chamber, it is best to de-air the rubber
before pouring it into the mold. De-airing prevents air bubbles from
forming in the mold. If a bubble is on the surface of the master, every
casting will have a raised "bump" at that spot. If you de-air the
rubber, it should be in a container that is three to five times as high as
the level of the rubber. The rubber will foam up to this height, then
settle back down as the air bubbles escape.
If you don't have a vacuum chamber, pour the rubber slowly in a thin
stream into the mold box. Pour it as far from the master as possible and
let it rise up over the master of its own accord. This is advisable even
if the rubber has been vacuum de-aired. The low durometer rubbers
usually have low pouring viscosities, and some of them (Ace Resin RTV,
Alumilite "Quick-Set" RTV, and Dow Corning HS-III & 3110) do not require
vacuum de-airing. Some RTV rubbers, such as Alumilite "Quick-Set," will
cure in as little as four hours, but for a long mold life it is best to let
the rubber cure for at least twenty-four hours before attempting to use
the mold. After the rubber has cured, carefully remove the clay from the
master and then carefully remove the master from the mold.
RTV Rubber Versatility
The low-durometer RTV rubbers permit one-piece molds to be made to cast
parts that would otherwise require multi-piece molds. For example, a
"faceted" Nike fin with a through-the-wall mounting tab can be cast in a
one-piece mold. The master pattern for the fin would be built-up using
cardstock, sheet styrene, or waferglass bonded to an internal framework
(see Peter Alway's "The Art of Scale Model Rocketry" (Saturn Press, 1994)
for details). A cardstock master should be sprayed with a wax-based mold
release. The mounting tab would be made longer than usual so that the
excess length could be pressed into a smooth bed of plasticene modeling
clay. A mold box would be built around the pattern and the RTV rubber
would be mixed and poured. After the rubber cured, the clay would be
removed from the mounting tab and the mold. To remove the master from
the mold, the mold's bottom (which would be inverted to pour a casting)
would be carefully slit with a modeling knife along a line passing down
the fin root.
Casting A Part
An RTV silicone rubber mold is self-releasing and requires no release
agent. However, mold life will be extended if a release agent is used.
A silicone-based or wax-based release agent may be used. Another good
release agent is talcum powder. It is especially useful in deep-draw
molds such as nose cone molds. Drop a few pinches of powder into the
mold, cover the opening, and shake it until the mold cavity is dusted
with the powder.
The resin is very easy to measure and mix. Most polyurethane resins (Ace
Resin, Alumilite, Por-A-Kast, etc.) are mixed 1:1 by volume. Equal
volumes of the two parts are poured into separate containers. The "A"
(or thinner) side can be poured into the "B" (or thicker) side and mixed, or
both sides can be poured into a third container and mixed. Stir
vigorously for 10-15 seconds or until a uniform color is achieved.
Slowly pour the resin into the mold. The resin should cure within 2-3
minutes. The curing resin will get quite hot (up to 215 degrees F for
Alumilite), so let the casting cool to room temperature before attempting
to remove it from the mold. The casting is still pliable while it is
warm, so it could be distorted if it is removed from the mold before it
cools to room temperature.
Fillers, Dyes, & Paints
The resins can be filled with any DRY filler (moisture reacts with the
resins and produces carbon dioxide bubbles) up to 100% by volume.
Aluminum powder can be added to give the cured part a metallic
appearance, or microbulb filler (microballoons) can be added to lighten
it so much that it would float on water. Several coloring dyes are
available from Alumilite. The cured resins can be painted, stained, and
The cured resins can be cemented to themselves and to kraft paper body
tubes, balsa, cardstock, other plastics, and fiberglass with epoxy and
cyanoacrylate adhesives. The resins can also be used as adhesives
themselves. They adhere to metal, soft wood, and plastic.
Casting One-Piece Nose Cones
To cast a solid, one-piece resin nose cone (with tenon) in a one-piece
mold, first fabricate a nose cone master as outlined above. Press it
into plasticene modeling clay, build a mold box around it, then mix and
pour the RTV rubber as described. Nose cones can be cast with a screw eye
already installed. Slide the screw eye's eyelet onto a dowel or a rolled
paper "stick." After pouring the liquid resin into the mold, hold the
dowel or "stick" by the ends and insert the screw eye into what will
become the back of the tenon, holding it steady until the casting
solidifies. A simple jig can also be made to hold the dowel steady. A
transition mold and casting would be made in the same way.
Molding A Two-Piece Hollow Nose Cone
Solid cast resin nose cones are fine for small models. At about 0.976"
(BT-50) size, however, even a microballoon-filled resin nose cone may be too
heavy to be practical. Using resin casting techniques, it is possible to
make lightweight two-piece hollow nose cones.
Two masters are required for the nose cone (with tenon) and for the nose
cone base--an outer shape and an inner "plug" for each piece. In the
case of the nose cone, the outer shape would be turned on a lathe or a
drill press. The plug would be turned in the same way, and it would have
the same shape as the outer shape. The plug would not have a tenon,
but rather a cylindrical section on the back that would have the same
length as the outer shape's tenon. The plug's radius should be at least
3/32" less than the radius of the outer shape, and the plug's length
would be proportionally less than that of the outer shape. The alignment
pins (either turned with the piece or a dowel cemented into the blank
before turning) on the plug and the outer shape will serve to center the
two pieces in the next step.
Take the outer shape and press its alignment pin into a smooth bed of
plasticene modeling clay. Press it in until the back of the tenon is
flush with the clay's surface. Use a dowel or other object to make
several depressions in the clay around the master, and build a mold box
around these. The rubber will flow into these holes and create keys
which will align the two mold halves. Mix and pour an RTV rubber as
described above and set it aside to cure. After the rubber has cured,
carefully remove the clay from the master and the mold, but leave the
master in the mold. Next, either brush a rubber-to-rubber mold release
ONLY on the exposed upper mold surface, or spray a wax-based mold release
on the upper mold surface and the exposed part of the master. After it
has dried, build a dam around the edges of the mold (sheet styrene taped
around the edges is sufficient). Mix another batch of RTV and pour it
onto the mold surface, then set it aside to cure. After the rubber has
cured, separate the two halves of the mold. Remove the outer shape from
the upper mold half.
Before casting a nose cone, insert the plug's alignment pin into the
upper mold half and push it all the way in. Spray the plug with a
wax-based release agent and allow it to dry. Mix the resin and pour it
into the mold cavity, then slowly press the upper mold half into place.
Allow the casting to cool to room temperature before removing it from the
The plug must be sprayed with the release agent before casting each nose
cone. If desired, a second all-RTV upper mold half can be cast. To do
this, leave the hollow cast resin nose cone in the mold cavity. Spray
the exposed upper mold surface with a wax-based release agent and let it
dry. Build a dam around the mold edges as before. Mix a batch of RTV
rubber and pour it onto the upper mold surface. Pour it away from the
hollow nose cone and let the rubber slowly run down into the nose cone.
After covering the upper mold surface (1/4" to 1/2" deep should be
enough) with the rubber, set it aside to cure. The new upper mold half will
have an integral plug. The nose cone base mold would be produced in the
same way, and transition and boat-tail molds would use similar techniques.
"Pseudo Blow-Molded" Hollow Nose Cones
There is an easier way to make hollow nose cones. It requires only one
master pattern. This method works best if the tenon has a truncated cone
on its rear like blow-molded nose cones do, hence its name. Turn the
master on a lathe or a drill press. It needs to have an alignment pin in
the center of the tenon rear (the truncated cone). Press the master into
a smooth bed of plasticene modeling clay, all the way up to the truncated
cone/tenon interface. Use a dowel or other object to make depressions in
the clay around the master, then build (or place) a mold box around
Because this casting method requires that the mold be held and rotated,
it must be protected from being squeezed during casting. There are two
ways to do this. The mold can be cast into a rigid cylinder (a length of
plastic pipe, for example) or into a rigid box. Another method is to
cast the mold in a temporary mold box, then brush on a thixotropic (a
thixotrope is a gel that will not run when applied to a surface) mold
support shell resin. Synair makes a mold support shell resin called
Mother Mold, and other formulations are available from other casting
suppliers. If you go this route, leave the master in the mold while
brushing on the mold shell support resin. The mold itself will be a
two-piece mold, and the piece covering the truncated cone will be the
smaller of the two. Brush the mold shell support resin onto the entire
mold except the top of the smaller piece. The cured mold support shell
will be a rigid cylinder or box, open at the top so that the upper mold
half can be removed.
Mix a batch of RTV rubber and pour it in a thin stream into the mold box.
Pour it away from the master and let the rubber slowly rise up over the
master. Set aside to cure. After the rubber has cured, carefully
remove the clay from the master and the mold, but leave the master in the
mold cavity. Brush a rubber-to-rubber mold release ONLY on the exposed
upper mold surface, or spray a wax-based release agent on the upper mold
surface and the exposed part of the master.
Build a dam around the upper edges of the mold (if you're not casting it
in a rigid cylinder or a rigid box). Mix a batch of RTV rubber and pour
it over the mold surface. Pour enough rubber that it rises almost to the
top of the alignment pin on the truncated cone, then set it aside to
cure. After the rubber has cured, separate the mold halves and carefully
remove the master from the mold. The opening in the upper mold half left
by the alignment pin is the channel into which the casting resin will be
To cast a nose cone, assemble the mold halves and insert the mold into
its cylinder, box, or shell. Cut a piece of pressure-sensitive adhesive
paper (or a pressure-sensitive mailing label) large enough to cover the
opening in the upper mold half. Cut or tear off a piece of masking
tape. The tape will be used to keep the upper mold half from falling
out of the box or shell during casting. Insert a funnel into the opening
in the upper mold half. Mix the casting resin and pour it into the
funnel, then quickly remove the funnel and press the pressure-sensitive
adhesive paper over the opening. Using the masking tape, tape the upper
mold half into place. Gently rotate the mold in all directions so that
the resin coats the entire surface of the mold cavity. Rotate the mold
until you can no longer hear the resin flowing inside, then set it aside
to cure. The casting will be rather thin, so make sure it has cooled to
room temperature before attempting to remove it from the mold. If
desired, leave the casting in the mold and repeat the casting process to
build up the casting's thickness. Hollow transitions and boat-tails can
be cast in the same way.
Molding & Casting Materials Suppliers
Several suppliers of molding and casting materials are listed in Section
02 of the rec.models.rockets Frequently-Asked-Questions (FAQ) archive.
The following is a list of manufacturers and distributors of resin
casting materials. These are from the rec.models.rockets FAQ, and I
snipped them in their "raw" form. Rearrange them as needed to fit the
rec.models.scale FAQ's format. I received permission from Wolfram v.
Kiparski, maintainer of the RMR FAQ, to send this to you. He agreed with
me that there can and should be more "cross-fertilization" of ideas
between our two groups. He said that you need not give acknowledgement
I've also included information on Saturn Press (Peter Alway) and on his
scale model rocketry books (his reviews of the Saturn V plastic kits are
in the RMS FAQ).
Here's the list:
Ace Resin Resin casting and molding supply
7481 E. 30th St. polyurethane resin and
Tucson, AZ 85710 RTV rubber, supplies
(520) 886-8051 Catalog - $3.00, or
#10 SASE for price list
Alumilite Corporation resin casting and molding supply
225 Parsons Street resins and supplies
Kalamazoo, MI 49007 will send literature, specs,
(616) 342-1259 and cured samples on request
(616) 342-1299 FAX
(Alumilite has a web site at www.alumilite.com)
Bare-Metal Foil and Hobby Co. Por-A-Kast, Por-A-Mold dealer
PO Box 82 urethane resin molding and
Farmington, MI 48332 casting, adhesive metal foil
(810) 477-0813 Days
(810) 476-4366 Evenings or messages
(810) 476-3343 FAX
The Castolite Company casting resins
4915 Dean St. polyester resins and rubber
Woodstock, IL 60098
(815) 338-4671 FAX
Micro-Mark Small tool company.
340 Snyder Ave. The company isn't small!
Berkeley Heights, NJ 07922-1595 Perfect tools for working on
(800) 225-1066 rockets.
(908) 665-9383 FAX
(Micro-Mark is also an Alumilite distributor)
Ogilvie, Inc. RBC Industries dealer
116 W. Santa Fe Ave. epoxy casting resins, fiberglass,
Placentia, CA 92870 RTV silicone rubber, adhesives
(714) 572-0641 FAX
Smooth-On, Inc. casting resins and rubber
1000 Valley Road polyurethane and epoxy resins
Gillette, NJ 07933
(908) 604-2224 FAX
Synair Corporation Resin casting and molding supply
2003 Amnicola Highway Por-A-Kast and Por-A-Mold
PO Box 5269 RTV rubber
Chattanooga, TN 37406-0269
(423) 698-8801 will send literature, specs, and
(800) 251-7642 cured samples on request
(423) 624-0321 FAX
(Synair has a web site at www.synair.com)
Scale Data Reference Books
Saturn Press Books on scale rocketry,
P. O. Box 3709 rocketry scale reference
Ann Arbor, MI 48106-3709 materials, a great rockets
(313) 677-2321 of the world poster.
(Peter Alway) Catalog: Free, also available
Rockets of the World, 2nd edition
Saturn Press, 1995
This book contains information on more than 200 versions of 137
rockets from 14 countries and Europe. An absolute must buy for
scale modeling enthusiasts and rocketry enthusiasts in general.
Available via mail order from Saturn Press, NARTS, Quest, Countdown
Hobbies, Magnum, Mountainside Hobbies and other sources.
Retro Rockets: Experimental Rockets 1926-1941
Saturn Press, 1996
More than 30 rockets flown by Robert Goddard and his
contemporaries, depicted with dimensioned drawings,
color-keyed drawings, and photographs. Historical
backgraound for each. 96-page hardcover.
The Art of Scale Model Rocketry
Saturn Press, 1994
A complete and well written guide to building scale model rockets
from scratch. Includes chapters on obtaining scale data,
building techniques, and finishing your model. Also includes NAR
scale competition tips. Complete plans for 13 different scale projects
are presented. 96 pages, wirebound.
Copyright (c) 1996 Wolfram von Kiparski, editor.
Refer to Part 00 for the full copyright notice.
Here's another resin casting materials firm I recently found:
Polytek Development Corporation is a big firm. They make dozens of
different RTV rubbers (tin and platinum cure), scores of polyurethane,
epoxy, polyvinyl, and hot-melt casting resins, plus fillers and molding &
casting accessories. They have a 60-page "Moldmaking & Casting, Methods &
Materials Manual & Catalog" that has illustrated instructions on how to
produce molds and castings, including molded fiberglass parts. It also
contains a glossary of the technical terms as well as material
compatibility tables (what resin works best with what rubber for what
application). This manual/catalog is $10.00 (free with an order), but I'd
consider it a bargain at twice the price.
Their address is:
Polytek Development Corporation
55 Hilton Street
Easton, PA 18042
(610) 559-8626 FAX
rec.models.scale FAQ, part 19
FAQ Table of Contents